U.S. Pat. No. 5,176,628 describes a pneumatically-controlled vitreotome. In that vitreotome, the sharp member is mounted to rotate inside the cannula. The reciprocating rotation of the sharp element is obtained by causing a piston to perform reciprocating motion under the control of pneumatic means. The piston has a rack that itself co-operates with a pinion that is secured to the rotary sharp tool.
Such a vitreotome suffers from the essential drawback of presenting an assembly for controlling movement of the sharp tool that is relatively complex and that includes a mechanical portion that makes obtaining accuracy over the movement of the sharp tool relatively random and that introduces a large amount of inertia in the control of the movement of the sharp tool of the vitreotome.
When performing a posterior vitreotomy, i.e. when intervening close to the retina, the vitreotome needs to ensure high-speed cutting with the length of time that the cannula is fully closed being as short and as stable as possible, even if the cutting speed is very high. The purpose of this is to avoid creating excessive suction inside the cannula which would run the risk of harming the retina on the next occasion the cannula is opened because of the large amount of suction that exists in the cannula.